Washington's Strategy for Protecting Reading and Philadelphia

Washington assumed that Howe would try to press his advantage and turn the American right flank, cut off a retreat to the west and force the Americans into the pocket beetween the Schuylkill and Delaware Rivers. Aware of this, but also cognizant that he would have to protect the upper fords — the likely crossing spot of the Schuylkill into Philadelphia — Washington had to choose a smart spot for defense.
The Americans knew that the Schuylkill south of Philadelphia was above flood level and would be too difficult for the British to cross. Moreover, the Americans had troops posted in forts along the river there.

The fording spots nearer to Philadelphia were protected by militia on the eastern side. Further, Washington ordered the bridge at Middle Ferry unmoored and swung toward the Philadelphia side to impede any possible British crossing there. Washington's subsequent position on the west side of the river would still give him enough time to move to defend Philadelphia if the British tried to march on Philadelphia.

This left the upper fords as the only possible means of crossing the Schuylkill. Washington placed his army directly in-between the British and those fords.